The invention relates to a fluorinated, rare earth doped glass composition that exhibits reduced rare earth ion clustering and fluorescence quenching in the presence of relatively high concentrations of rare earth dopants, and to ultratransparent glass-ceramic articles, particularly active fiber waveguides and devices, e.g., fiber amplifiers and lasers, that utilize such waveguides. As used herein, the term xe2x80x9cultratransparentxe2x80x9d refers to high optical clarity; i.e., the transparency of the glass-ceramic of the invention compares to the transparency of glass over the spectrum from about 350 nm to 2.5xcexc.
There is widespread interest in, and industrial need for, optical compositions and articles made therefrom having potential applications in the 1300 nm and 1550 nm telecommunications windows. Promising candidates for an efficient 1300 nm fiber optical amplifier material, for example, have included the rare earth ions Pr3+ and Dy3+ doped in fluoride, mixed halide, and sulfide glass hosts, while 1550 nm amplifier materials are suitably doped with Er3+. A recent publication by Borrelli et al., Transparent glass ceramics for 1300 nm amplifier applications, J. Appl. Phys. 78 (11), (Sept. 1995), reported an alternative host for the Pr3+ ion which combines some of the advantages of both fluoride and oxide glasses. The new material is described in the ""505 patent and consists of an oxyfluoride glass which has been appropriately heat treated to form a transparent glass-ceramic. This glass-ceramic contained 5-40 volume % fluoride nanocrystals having diameters ranging between about 6-15 nm, embedded in a primarily oxide glass matrix. As described in detail in the ""1505 patent, optically active fluoride based glass-ceramic articles were produced from Yb-free compositions that included between about 50 to 900 ppmw Pr3+. The glass-ceramic was shown to perform as an active device in the 1300 nm spectral window over this dopant concentration range. For the Pr3+ doped glass-ceramics, fluorescence lifetimes greater than 120 microseconds were observed in the base glasses of the ""505 patent at Pr3+ concentrations up to about 500 ppmw. Concentration quenching was observed at Pr3+ concentrations slightly above 500 ppmw, and fluorescence lifetimes were observed to decrease approximately linearly to about 70 microseconds at 900 ppmw. It was reported that a best case balance between fluorescence lifetime and concentration was achieved with Pr3+ in the range of about 200 to 550 ppmw; however, functional active devices were reported with Pr3+ concentrations in the range of about 50 to 650 ppmw. Since both longer fluorescence lifetimes and higher dopant concentrations are desirable for the production of active devices as described herein, the inventors recognized a need to improve upon the compositional ranges of the new glass-ceramic material described in the ""505 patent, and to devise glass-ceramic compositions having similar advantages suitable for 1550 nm applications.
The radiative quantum efficiency is a key parameter in evaluating transparent glass-ceramics as a potential gain medium for fiber lasers and amplifiers. Quimby and Tick, in an article entitled Quantum efficiency of Pr3+ doped transparent glass-ceramics (to be published) report on the quantum efficiency of the 1300 nm emission in Pr3+ doped transparent glass-ceramics using a direct measurement technique based upon relative fluorescence measurements. Fluorescence was observed by exciting the Pr3+ 1D2 level, peaking at around 1460 nm (the xe2x80x9cAxe2x80x9d transition), and the fluorescence from the 1G4 level, peaking at about 1300 nm (the xe2x80x9cBxe2x80x9d transition), when the 1D2 level was directly excited with 595 nm dye laser radiation. Following the analysis described by Quimby et al., Opt. Left., 20, 2021 (1995), the quantum efficiency of the 1G4 1300 nm emission was determined by taking the ratio of the total B transition rate to the total A transition rate. The data in FIG. 1, to be described in more detail below, shows the measured B/A ratio for exemplary embodiments of the two base composition glass-ceramics of the ""505 patent having Pr3+ concentrations ranging from about 25 ppmw to 1000 ppmw. As expected by the inventors, the B/A ratio increases with increasing concentration which they believe to be due to the effect of cross-relaxation resulting from increased Pr3+ ion clustering.
It is known that when a trivalent rare earth, e.g., Pr3+, is incorporated into these glass-ceramics, the rare earth is segregated into the second phase crystals which are formed during the ceraming process. These crystals have a cubic lattice structure and are thought to be comprised of mostly divalent cadmium- and lead-fluoride. The inventors believe that clustering arises from local strains that are established within the lattice because of the substitution of trivalent rare earth fluorides for the divalent fluorides. When direct substitution of a rare earth into the crystal lattice occurs, charge balance can be maintained by incorporating an interstitial fluorine into the crystal structure near the rare earth. In bulk crystals this is the source of the local strain, which is observed to decrease when these defects can cluster. The inventors believe that a similar mechanism occurs in the nanocrystals of the glass-ceramic. This, however, results in a decrease in the quantum efficiency at higher concentrations of Pr, observed by the authors to appear at concentrations of about 500 ppmw.
The inventors have therefore recognized a need for transparent rare earth doped glass and glass-ceramic compositions and articles made therefrom in which rare earth ion clustering and concentration quenching are reduced notwithstanding high rare earth dopant concentrations, which have a relatively high quantum efficiency, and a wider spectral gain band.
Accordingly, the invention is directed to a glass-ceramic optical article having a glass composition providing such features, and to a method for making glass-ceramic optical fiber waveguide articles having core and cladding compositions as described herein.
An embodiment of the invention is directed to a glass-ceramic optical article. The compositional structure of the glass-ceramic article is a second phase cubic lattice substantially including either divalent cadmium fluoride or divalent lead fluoride having a trivalent rare earth ion incorporated therein, and including a monovalent silver or monovalent thallium ion to charge balance the crystal.
Another embodiment of the present invention that is particularly suitable for applications in the 1300 nm telecommunications window describes a glass-ceramic optical article including an active core that is a transparent glass-ceramic having substantially only one crystal phase, consisting essentially, in terms of cation percent, of: SiO2 20-40; AlO1.5 10-20; CdF2 19-34; PbF2 19-23; wherein up to 5 mole % of CdS or 3 mole % of CdCl2 can be substituted for an equivalent amount of CdF2, or an equivalent amount of an oxide can be substituted for the fluoride; and including at least one of the rare earth fluorides YF3 (3-7), GdF3 (3-7), and LuF3 (4-15) wherein the total amount of these rare earth fluorides is (3-15); including at least one of Pr3+ and Dy3+ at a concentration in the range of about 300 to 2,000 ppmw; and including Ag+ at a concentration in the range of about 500 to 2,000 ppmw; and a cladding that is a transparent glass, consisting essentially, in weight percent on an oxide basis, of: SiO2 25-35; Al2O3 3-5; CdF2 12-16; PbF2 40-50; ZnF2 4-8; and Bi2O3 0-10.
Another embodiment of the present invention that is particularly suitable for applications in the 1300 nm telecommunications window describes a glass-ceramic optical article including an active core that is a transparent glass-ceramic, having substantially only one crystal phase, consisting essentially, in terms of cation percent, of: SiO2 20-40; PbF2 15-25; AlO1.5 10-20; CdF2 21-31; ZnF2 3-7; wherein up to 5 mole % of CdS or 3 mole % of CdCl2 can be substituted for an equivalent amount of CdF2, or an equivalent amount of an oxide can be substituted for the fluoride; and including at least one of the rare earth fluorides YF3 (3-7), GdF3 (3-7), and LuF3 (4-15) wherein the total amount of these rare earth fluorides is (3-15); including at least one of Pr3+ and Dy3+ at a concentration in the range of about 300 to 2,000 ppmw; and including Ag+ at a concentration in the range of about 500 to 2,000 ppmw; and a cladding that is a transparent glass, consisting essentially, in weight percent on an oxide basis, of: SiO2 25-35; Al2O3 3-5; CdF2 12-16; PbF2 40-50; ZnF2 4-8; and Bi2O3 0-10.
In an aspect of the above described embodiments, the Ag+ is at a concentration in the range of about 700 to 1,000 ppmw.
Another embodiment of the invention that is particularly suited for applications in the 1550 nm telecommunications window describes a glass-ceramic optical article including an active core that is a transparent glass-ceramic having substantially only one crystal phase, consisting essentially, in terms of cation percent, of: SiO2 20-40; AlO1.5 10-20; CdF2 19-34; PbF2 19-23; wherein up to 5 mole % of CdS or 3 mole % of CdCl2 can be substituted for an equivalent amount of CdF2, or an equivalent amount of an oxide can be substituted for the fluoride; and including at least one of the rare earth fluorides YF3 (3-7), GdF3 (3-7), and LuF3 (4-15) wherein the total amount of these rare earth fluorides is (3-15); ErF3 at a concentration in the range of about 500 to 5,000 ppmw; and including Ag+ at a concentration in the range between zero to 2,000 ppmw; and a cladding that is a transparent glass, consisting essentially, in weight percent on an oxide basis, of: SiO2 25-35; Al2O3 3-5; CdF2 12-16; PbF2 40-50; ZnF2 4-8; and Bi2O3 0-10.
Another embodiment of the invention that is particularly suited for applications in the 1550 nm telecommunications window describes a glass-ceramic optical article including an active core that is a transparent glass-ceramic having substantially only one crystal phase, consisting essentially, in terms of cation percent, of: SiO2 20-40; PbF2 15-25; AlO1.5 10-20; CdF2 21-31; ZnF2 3-7; wherein up to 5 mole % of CdS or 3 mole % of CdCl2 can be substituted for an equivalent amount of CdF2, or an equivalent amount of an oxide can be substituted for the fluoride; and including at least one of the rare earth fluorides YF3 (3-7), GdF3 (3-7), and LuF3 (4-15) wherein the total amount of these rare earth fluorides is (3-15); ErF3 at a concentration in the range of about 500 to 5,000 ppmw; and including Ag+ at a concentration in the range between zero to 2,000 ppmw; and a cladding that is a transparent glass, consisting essentially, in weight percent on an oxide basis, of: SiO2 25-35; Al2O3 3-5; CdF2 12-16; PbF2 40-50; ZnF2 4-8; and Bi2O3 0-10.
In an aspect of all of the above described embodiments, the core is an elongated central member having a first and a second end, and the cladding covers the surface of the elongated central member but leaves exposed the first and second ends.
In another aspect of all of the above described embodiments, the core composition contains up to 17 cation percent total of at least one component selected from the group consisting of (0-7%) BO1.5, (0-12%) GeO2, (0-7%) PO2.5, (0-3%) TiO2,(0-2%) Nb2O5, (0-7%) GaF3, (0-7%) HfF4, (0-7%) lnF3, (0-15%) BiF3, (0-1%) LaF3, (0-3%) CdCl2, and (0-5%) CdS.
In an aspect of all of the embodiments recited above, silver is in the form of a monovalent cation provided by, e.g., silver fluoride (AgF), silver oxide (Ag2O), silver nitrate (AgNO3), or any common silver salt
Another aspect of the invention pertains to a method for making an optical fiber waveguide comprising the steps of charging an inner crucible of a double crucible furnace with a finished core glass composition in a fluid state, preferably in the form of remelted cullet, providing a cladding glass having a sufficient stiffness to contain the fluid core, preferably in the form of a tube, in an outer crucible of the double crucible furnace, maintaining the core and cladding glasses at a temperature at or above their respective liquidus temperatures such that no portion of the core or cladding glasses above their respective liquidus temperatures comes into contact with a platinum wall of the double crucible; extracting an elongated glass article from the furnace, and cooling the glass article to below its liquidus temperature. In an aspect of the embodiment, the center member glass and the cladding glass are heated to a temperature in the range of about 800-1300xc2x0 C., and the elongated glass article is quenched to a temperature below the peak crystallization temperature in a time of less than 1 minute.
The elongated glass article, e.g., an optical fiber, preferably has a first and a second end, and has a core and cladding composition described in one of the above recited embodiments. The core of the glass article can be transformed into a transparent glass-ceramic having high optical clarity and containing essentially only one crystal phase by heating the elongated glass article at a pre-selected temperature for a pre-selected time. Preferably, the ceramming step is carried out by heating the glass article to near the peak crystallization temperature of the central member glass for between about xc2xd-24 hours. By the expression xe2x80x9csubstantially one crystal phase,xe2x80x9d it is meant that the glass-ceramic does not contain a sufficient amount of a second crystal phase to alter the chemical and/or physical characteristics of the glass-ceramic, most particularly, the optical clarity. Most preferably, no amount of a second crystal phase will be present. The rare earth metal ions are present in the crystal phase(s).
In an aspect of the invention in which the core glass composition contains up to 5 mole % of CdS or 3 mole % of CdCl2 substituted for an equivalent amount of CdF2, or has an equivalent amount of oxide substituted for fluoride, the glass core is transformed into a glass-ceramic upon cooling as the article leaves the furnace, and no additional or external ceramming step is required.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the apparatus and method particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.